Method and apparatus for expanding cell coverage in mobile communication system

ABSTRACT

A method, apparatus and storage mediums executed by a computer and storing the method, for expanding cell coverage in mobile communication system is disclosed. The method comprises the steps of: a) shifting a preamble window by delaying a transmission signal in order to catch a first call access signal from a mobile station at a remote distance; and b) delaying a second call access signal from a mobile station at a short distance in order to catch the second call access signal.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for expandingcell coverage in a mobile communication system; and more particularly,relates to a method and apparatus for expanding cell coverage byadvancing a transmission signal and selectively delaying a receivedsignal in code division multiple access (CDMA) system.

PRIOR ART OF THE INVENTION

FIG. 1 shows a general CDMA mobile communication system.

A mobile station (MS) 11 communicates a call setup signal and a trafficsignal with a base station (BS) 12 through a radio path and performssignal processing function for speech conversion and called/callingspeech.

The BS 12 performs communication of the call setup signal and thetraffic signal with the MS 11 through the radio path and a call controlfunction. The BS 12 is coupled to a base station controller (BSC) 13with a wired line and transmits the call setup and the traffic signalsfrom the MS 11 to the BSC 13 and message from the BSC 13 to the MS 11.Also, the BS 12 performs a signal processing function for communicationwith the BSC 13 and maintenance function. The BS 12 provides services inat least one or more sectors in which a cell is divided a certain numberof sectors.

The BSC 13 is coupled to a plurality of the BSs 12 and a mobileswitching center (MSC) 14. The BSC 13 performs called/calling processingand handoff of the call, and management and maintenance of the BS.

The MSC 14 is coupled to the BSC 13, a public switchingtelecommunication network (PSTN) and another MSC 14. The MSC 14 performscalled/calling processing, handoff and management/maintenance for amobile communication system.

In a general mobile communication system, a plurality of the basestations 12 are placed in a service area having a plurality of cells,regarding efficient frequency allocation and power consume etc. The cellserviced by one BS 12 includes a certain number of sectors. When the MS11 moves to another sector or another cell on calling, the MS 11 iscontinuously connected to another base station 12 or a radio channel ofanother sector (which is referred to ‘handoff’).

Hereinafter, access parameters between the MS and the BS in the generalmobile communication system will be described.

Since a signal delayed by approximately 1.5 or more Walsh symbol (cellradius of 45 Km) cannot be participated in demodulation, size of theaccess parameter for access probe is limited.

Preamble size means a size of preamble contained in a access channelslot. Here, a minimum length of one access channel slot is 4 frames anda maximum length is 26 frames. The preamble size is between 1 and 16frame(s).

When increasing the preamble size, the BS 12 can accurately captureaccess of the BS. Also, a length of the preamble access window can beincreased, and then a preamble integration period can be reduced.However, overhead is increased, thereby there are some problems that acall setup time is increased and a size of message capsule of the accessprobe can be reduced. Here, the length of the preamble access windowmeans a size of a search window when the BS 12 detects the preamble ofthe access probe. The length of the preamble access window ranges from 1to 3072, in other words, 1/8 pseudo noise chip.

If a round trip delay (RTD) of a signal is 1.5 or more Walsh symbol, theBS 12 cannot process the signal, thereby the RTD being limited to themaximum 384 chips. Here, one (1) Walsh equals to 256 chips. Therefore,when the preamble access window size is large, though a preamble pseudonoise random delay is large, the preamble can be detected and radius ofreverse coverage can be increased. However, the preamble integrationperiod should be short, thereby detection of accurate preamble beingdifficult.

The preamble integration period means time necessary to search an accessprobe from the MS 11 in an access channel. If the preamble integrationperiod is small, it is difficult an accurate preamble to be detected.However, the length of preamble access window can be increased becauseof reduction of preamble searching time. Therefore, in order toaccurately search the preamble by enlarging the preamble integrationperiod, the length of the preamble access window should be reduced, orthe size of the preamble should be enlarged.

A base station channel card includes four base station demodulators(BSD). A combiner performs software combining by interrupting itsoperation at every 1 Walsh symbol on basis of a global positioningsystem (GPS) time and reading symbols from the four BSDs.

Here, each of the BSD demodulates a signal received through a differentpath, which arriving time of the signal is different from each other.Therefore, a finger of the BSD cannot demodulate a signal delayed bydeskewing time or more than. In other words, the remote MS more than 1.5Walsh chip distance cannot access to the BS. The reason is that thesignal delayed by the deskewing time can have a different value from adesired signal in case of 1.5 or more than Walsh symbols (PN 384 chipsor a radius of 45 km).

Therefore, when demodulating, the window size for detecting the accessprobe of the MS 11 is limited to PN 384 chips, and then a radius ofservice area of the BS 12 is limited to about 45 km.

Most effective one to the cell coverage of the BS 12 in parameters fordetecting a call access from the MS 11 is the window size for accessprobe of the MS 11. Here, the window size is determined according to apreamble pseudo noise offset PRA_(—)PN_(—)Offset and the length of thepreamble access window PRA_(—)WIN_(—)LEN.

The preamble pseudo noise offset PRA_(—)PN_(—)Offset means an initialvalue of pseudo noise (PN) offset searched by the BS 12 when the BS 12searches the preamble of the channel access probe, and a minimumprocessing delay time of the MS 11 and the BS 12. The preamble pseudonoise offset ranges from 0 to 3072 PN chip(s).

The length of the preamble access window PRA_(—)WIN_(—)LEN means thesize of the searching window when the BS 12 detects the preamble of theaccess probe. The length of the preamble access window ranges from 1 to3072 PN chip(s).

The access probe timing is illustrated in FIG. 2.

FIG. 2 is a timing diagram illustrating access probe timing in the BS inaccordance with the prior art.

When the BS transmits the signal to the MS, if the MS locates within theservice coverage of the BS, in other words, if the MS is a short distantor a middle distance from the BS, a round trip delay is acquired withinthe preamble access window, thereby a call can be set. However, if theMS is a remote distance from the BS, the round trip delay cannot beacquired within the preamble access window, thereby the call cannot beset. As can be seen, if the MS is a remote distance from the BS so thatthe round trip delay of the signal is not acquired, even though areceived power is large enough, a communication service can not beprovided.

As mentioned above, if the cell radius in which reverse access probe canbe performed is large in a conventional CDMA system and the round tripdelay of the signal is large, in other words, the call timing is out ofthe window size, a call cannot be done.

Therefore, the radius of the cell coverage is limited to about 45 Km inthe conventional CDMA system.

SUMMARY OF THE INVENTION

It is an object to provide a method and apparatus for expanding cellcoverage by advancing a transmitting signal and selectively delaying areceived signal in a mobile communication system.

In accordance with an aspect of the present invention, there is provideda method for expanding cell coverage in mobile communication systemcomprising the steps of: a) shifting a preamble access window byadvancing a transmission signal by a first delay time in order toacquire a first call access signal from a mobile station at a remotedistance; and b) delaying a second call access signal from a mobilestation at a short distance by a second delay time in order to acquirethe second call access signal.

In accordance with another aspect of the present invention, there isprovided a method for expanding cell overage in mobile communicationsystem including at least a ase station and a plurality of mobilestations, the method comprising the steps of: a) expanding a length of acurrent preamble access window to a predetermined value; b) determiningthe mobile station is within a cell coverage covered by the preambleaccess window; c) shifting the preamble access window by advancing atransmission signal by a first delay time in order to acquire at lease amobile station when the mobile station is out of the cell coverage; andd) delaying a second call access signal from a mobile station at a shortdistance in order to acquire the second call access signal.

In accordance with further another aspect of the present invention,there is provided an apparatus for expanding cell coverage in mobilecommunication system including a plurality of mobile stations and atleast a base station, comprising: transmitting means for transmitting atransmission signal advanced by a first delay time in order to acquirethe transmission signal from a mobile station at a remote distance; andreceiving means for receiving and delaying a received signal from amobile station at a short distance by a second delay time in order toacquire the received signal.

In accordance with still another aspect of the present invention, thereis provided a method for expanding cell coverage applied to a mobilecommunication system, the method comprising the steps of: a) expanding alength of a current preamble access window to a maximum value; b)setting operation parameters related to time advance in order to acquirea first call access signal from a mobile station at a remote distancewhen the mobile station is out of the cell coverage; and c) acquiring asecond call access signal from a mobile station at a short distancewithin the preamble access window by delaying the second call accesssignal.

In accordance with still another aspect of the present invention, thereis provided a method for expanding cell coverage applied to a mobilecommunication system, the method comprising the steps of: a) expanding alength of a current preamble access window for access probe to a maximumvalue; b) shifting the preamble access window by setting operationparameters in order to acquire a first call access signal from a mobilestation at a remote distance when the mobile station is out of theexpanded preamble access window; and c) acquiring a second call accesssignal from a mobile station at a short distance within the preambleaccess window by delaying the second call access signal by a feedbackdelay which makes the second call access signal have a different delayvalue on basis of a distance between a location of the mobile stationand a location on a cell radius.

In accordance with still another aspect of the present invention, thereis provided a computer readable media storing a method for expandingcell coverage applied to a mobile communication system, the methodcomprising the steps of: a) expanding a length of a current preambleaccess window to a maximum value; b) setting operation parameters inorder to acquire a first call access signal from a mobile station at aremote distance when the mobile station is out of the cell coverage; andc) acquiring a second call access signal from a mobile station at ashort distance within the preamble access window by delaying the secondcall access signal.

In accordance with still another aspect of the present invention, thereis provided a computer readable media storing a method for expandingcell coverage applied to a mobile communication system, the methodcomprising the steps of: a) expanding a length of a current preambleaccess window for access probe to a maximum value; b) shifting thepreamble access window by setting operation parameters in order toacquire a first call access signal from a mobile station at a remotedistance when the mobile station is out of the expanded preamble accesswindow; and c) acquiring a second call access signal from a mobilestation at a short distance within the preamble access window bydelaying the second call access signal by a feedback delay which makesthe second call access signal have a different delay value on basis of adistance between a location of the mobile station and a location on acell radius.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the instant invention willbecome apparent from the following description of preferred embodimentstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of a general CDMA mobile communication system;

FIG. 2 is a timing diagram illustrating access probe timing in the BS inaccordance with the prior art;

FIG. 3A is a block diagram of a transmitting unit in accordance with anembodiment of the present invention;

FIG. 3B is a diagram illustrating a method for expanding a cell coverageusing an adjusted transmission time and a delay element in accordancewith an embodiment of the present invention;

FIG. 4 is a block diagram of a receiving unit having a delay in areverse link in accordance with an embodiment of the present invention;

FIG. 5 is a flow chart illustrating a method for expanding a cellcoverage in accordance with an embodiment of the present invention;

FIGS. 6A and 6B are block diagrams of receiving units having feedbackdelays in accordance with other embodiments of the present invention;

FIGS. 7A to 7C are explanative diagrams illustrating a method forexpanding a cell coverage by using a feedback delay in accordance withanother embodiment of the present invention; and

FIG. 8 is a flow chart illustrating a method for expanding a cellcoverage by using a feedback delay.

MODES FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 3A is a block diagram of a transmitting unit in accordance with anembodiment of the present invention.

An original signal to be transmitted goes through transmission signalprocessing, for example, encoding, interleaving, etc. by a transmissionprocessor 31. The function of the transmission processor 31 is wellknown to ordinary one skilled in the art, therefore, detaileddescription will be skipped.

After the transmission signal processing is performed, the signal ismodulated by a pseudo noise (PN) modulator 32. In the whole processbefore transmitting signals through an antenna, processing time delay isinevitable. In order to compensate the processing delay, a parameter, atransmission time advance fine adjust value Tx_(—)Fine_(—)Adj is used.The PN modulator 32 performs PN modulation of the signal from thetransmission processor 31 on the basis of the transmission time advancefine adjust value Tx_(—)Fine_(—)Adj from a transmission time fineadjuster 33. The PN modulated signal is up converted by a frequency upconverter 34 and transmitted through an antenna.

The transmission time advance fine adjust value Tx_(—)Fine_(—)Adj is aparameter used for compensation of unavoidable time delay caused by thewhole signal processing such as PN modulation, interleaving, frequencyup converting and etc. In CDMA system, if a signal is delayed when thesignal is PN modulated, the signal has a result in being advanced.Therefore, adjusting the transmission time advance fine adjust valueTx_(—)Fine_(—)Adj of the signal makes the signal possible to be acquiredwithin the preamble access window.

If a preamble access window length is maximum, i.e., a chip length is3072 and a cell radius is limited to 45 Km, a transmission time advancefine adjust value Tx_(—)Fine_(—)Adj is changed in order to expand thecell coverage in the present invention. The transmission time advancefine adjust value Tx_(—)Fine_(—)Adj is expressed as following:$\begin{matrix}{{{Tx\_ Fine}{\_ Adj}( {\mu\; s} )} = {\frac{{{Radius}({Km})} \times 2}{3 \times 10^{5}} \times 10^{6}}} & \lbrack {{Eq}.\mspace{14mu} 1} \rbrack\end{matrix}$

Here, Radius is a length of the expanded radius and a speed of a radiowave is supposed to be equal to the speed of the light.

FIG. 3B is a diagram illustrating a method for expanding a cell coverageusing an adjusted transmission time and a delay element in accordancewith an embodiment of the present invention.

As described above, if the preamble access window is shifted by thetransmission time advance fine adjust value Tx_(—)Fine_(—)Adj, a mobilestation (MS) at a remote distance can communicate with the BS. However,a MS at a short distance can fail to communicate with the BS. Forpreventing the MS at a short distance from failing in communication withthe BS, a received signal is delayed by a delay 35 which is added in areceiving unit of the BS, such that the signal from the MS at a shortdistance is acquired within the preamble access window.

A delay amount of the delay 35 is expressed as following:$\begin{matrix}{{{Delay}\mspace{14mu}{{amount}( {\mu\; s} )}} = {{{Tx\_ Fine}{\_ Adj}( {\mu\; s} )} = {\frac{{{Radius}({Km})} \times 2}{3 \times 10^{5}} \times 10^{6}}}} & \lbrack {{Eq}.\mspace{14mu} 2} \rbrack\end{matrix}$

A receiving unit having a delay in a reverse link is illustrated in FIG.4.

Referring to FIG. 4, a signal from a remote distance is received by areceiving element Rx1 and a signal from a short distance is received bya receiving element Rx0 and delayed by a delay 31. Therefore, the signalfrom a short distance can be acquired by a shifted access window byadjusting the transmission time advance fine adjust valueTx_(—)Fine_(—)Adj. The signals from the MSs at the short distance andthe remote distance are combined in a diversity combiner 36 andperformed a receiving processing by a receiving processor 37, which iswell known to ordinary one skilled in the art, for exampledeinterleaving, decoding, etc. Therefore, a detailed description of thereceiving processor 37 will be skipped in here.

FIG. 5 is a flow chart illustrating a method for expanding a cellcoverage in accordance with an embodiment of the present invention.

In this embodiment, a signal from a MS at a remote distance further than5 Km is acquired by shifting back a window for access probe. In thisembodiment, though a round trip delay (RTD) of a signal is larger than45 Km, the window for the access probe can acquire the signal. Here, asignal from the MS at the short distance cannot be acquired because ofshifting the access probe window. Therefore, The signal from the shortdistance is delayed by a delay 31 in FIG. 4, thereby being accessed tothe access probe window.

In order to expand a cell radius limited to 45 Km in a general CDMAsystem, first, a preamble access window length is set as a maximumvalue, i.e. 3072 at step 501. In this embodiment, the preamble accesswindow length should not have a maximum value, however, for expandingthe cell coverage, the preamble access window length is preferred to beset as a maximum value.

At step 502, the preamble access window is shifted by setting parametersrelated with time advance, e.g., the preamble PN offsetPRA_(—)PN_(—)Offset and the transmission time advance fine adjust valueTx_(—)Fine_(—)Adj. The transmission time advance fine adjust valueTx_(—)Fine_(—)Adj for a call connection is set as the equation 1.

The proceed continues to step 503 to determine whether a round tripdelay (RTD) of the received signal from the MS is acquired within thepreamble access window for the access probe of the system.

If the RTD of the received signal is not acquired, in other words, ifthe MS is a short distance from the BS, the received signal is delayedby the delay 35 in the receiving unit Rx0 at step 504. Then, the delayedreceived signal is acquired within the preamble access window andaccessed at step 506.

If the RTD of the received signal is acquired, in other words, if the MSis a remote distance from the BS, the signal received by the receivingunit Rx1 without delay is used for the preamble access at step 505 andthen is acquired within the preamble access window and accessed at step506.

As described above, if the preamble access window is shifted, a signalfrom a remote distance can be acquired preamble access window within thepreamble access window. By delaying a signal from a MS at a shortdistance, which is not acquired, the signal can be acquired within thepreamble access window.

However, when expanding the cell coverage by using a general delay, thecell coverage is expanded as larger; as more delays are necessary inaccordance with the distance between a point in the cell and a point onthe cell radius. If there is no necessary number of delays, aprobability of call setup fail can increase. Therefore, a method tosolve this problem will be described in another embodiment of thepresent invention.

FIGS. 6A and 6B are block diagrams of receiving units having a feedbackdelay in accordance with other embodiments of the present invention.

Referring FIG. 6A, a signal from a MS at a remote distance is receivedby a receiving element Rx1 and a signal from a short distance isreceived by a receiving element Rx0 and delayed by a feedback delay 61.A delay value of the feedback delay 61 is set, the signal received bythe receiving element Rx0 is delayed by a number received from a gaincontroller. The signal received through the receiving element Rx0 isdelayed by the feedback delay number N (here, N=0, 1, 2, . . . ) andoutputted to a diversity combiner 63.

Using the feedback delay 61, the cell coverage can be expandedlimitlessly only if a forward or reverse transmission power is largeenough. By adding a delay to the receiving element Rx1, a region inwhich most of mobile stations are gathered can be covered, therebyproviding a stable call.

The signals received by the receiving elements Rx0 and Rx1 are combinedin a diversity combiner 63, demodulated by a channel demodulator 64 andperformed a receiving processing by a receiving processor 65, which iswell known to ordinary one skilled in the art, for exampledeinterleaving, decoding, etc. Therefore, a detailed description of thereceiving processor 37 will be skipped in here.

Referring to FIG. 6B, there is a different receiving element from thereceiving unit of FIG. 6A. The receiving unit has the same constitutionas one in FIG. 6A except that there is one receiving element Rx.Therefore, detailed description about the receiving unit in FIG. 6B willbe skipped.

FIGS. 7A to 7C are explanative diagrams illustrating a method forexpanding a cell coverage by using a feedback delay in accordance withanother embodiment of the present invention.

Referring to FIG. 7A, a cell coverage is illustrated when there is nodelay in the receiving unit of the base station. Referring to FIGS. 7Band 7C, cell coverages respectively expanded by 90 Km and 135 Km areillustrated. The cell coverage is expanded by controlling parameters forthe base station, e.g., the preamble PN offset PRA_(—)PN_(—)offset andthe transmission time advance fine adjust value Tx_(—)Fine_(—)Adj anddelaying the received signal by using a delay such as the delay 31described in FIG. 3.

Referring to FIG. 7C, the cell coverage is expanded by 90 Km or morethan 90 Km. Since one delay element can delay only the signal between 0and 45 Km, the signal between 45 Km and 90 Km cannot be acquired withinthe preamble access window.

Therefore, the signal in a range from 0 to 45 Km has a different delayvalue from the signal in a range from 45 to 90 Km. In order to solvethis problem, use of a feedback is delay such like in FIG. 6A ispreferred than use of a plurality of delay elements. The feedback delayhaving a minimum delay value is placed in the receiving unit and thereceived signal is delayed by N numbers of the minimum delay value, andthen the signal has a delay value to be wanted.

In case described above referring to FIG. 7C, a delay value which asignal in a range from 45 to 90 Km should have is set as a delay valueof the feedback delay. A signal in a range from 90 to 135 Km is passedwithout delay, the signal in a range from 45 to 90 Km is delayed by onetime and a signal in a range from 0 to 90 Km is delayed by two times,the signals in whole of the cell coverage can be acquired.

FIG. 8 is a flow chart illustrating a method for expanding a cellcoverage by using a feedback delay.

Referring to FIG. 8, a delay value of a feedback delay and a number N offeedback are initialized as ‘0’ at step 801. A preamble access windowlength is set as a maximum value, i.e. 3072, at step S01, the cellcoverage is expanded as 45 Km. In this embodiment, the preamble accesswindow length should not have a maximum value, however, for expandingthe cell coverage, the preamble access window length is preferred to beset as a maximum value.

After expanding the cell coverage at step 802, the preamble accesswindow is shifted by setting parameters related with time advance, e.g.,the preamble PN offset PRA_(—)PN_(—)Offset and the transmission timeadvance fine adjust value Tx_(—)Fine_(—)Adj at step 803. Thetransmission time advance fine adjust value Tx_(—)Fine_(—)Adj for a callconnection is set as the equation 1.

A signal from the mobile station (MS) is received by the receivingelements Rx0 and Rx1 in the base station at step 804. The signalreceived by the receiving element Rx1 is passed and transmitted to thediversity combiner. The signal received by the receiving element Rx0 isdelayed in the feedback delay by a certain time, in this embodimentabout 20 μs at step 805. The signal delayed by the feedback delay isrepeatedly delayed responsive to a control signal from the gaincontroller at step 805. The gain controller controls that the gain ofthe received signal becomes equal to a reference gain.

The proceed continues to step 806 to determine whether a round tripdelay (RTD) of the received signal from the MS is acquired within thepreamble access window for the access probe of the system.

If the RTD of the received signal is acquired, a call is setup at step807. If not, a call is not setup at step 808.

Using the apparatus and method in accordance with the present invention,the cell coverage can be expanded without additional equipment in anarea in which a base station is difficult to be established, e.g., seaand mountains, thereby the cost for establishing system can be reduced.Also, even though the cell radius is very large, there is no need to usea plurality of delay elements and only one feedback delay is necessaryto be used. Therefore, the cost can be considerably reduced.

Although the preferred embodiments of the invention have been disclosedfor illustrative purpose, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

1. A method for expanding cell coverage in a code division multipleaccess (CDMA) mobile communication system comprising the steps of: a)shifting a preamble access window by advancing transmission time of atransmission signal by a first delay time in order to acquire a firstcall access signal from a mobile station at a remote distance; and b)delaying a second call access signal from a mobile station at a shortdistance by a second delay time in order to acquire the second callaccess signal, wherein the transmission time of the transmission signalis advanced by delaying the transmission signal when being modulated;and wherein the first delay time is substantially equal to the seconddelay time, and the first delay time is expressed as:${{{first}\mspace{14mu}{delay}\mspace{14mu}{{time}({\mu s})}} = {\frac{{{Radius}({Km})} \times 2}{3 \times 10^{5}} \times 10^{6}}},$where Radius is a length of the expanded radius.
 2. The method asrecited in claim 1, wherein said step a) includes the step of expandinga length of the preamble access widow to a maximum value by adjusting alength of chips included in the transmission signal.
 3. The method asrecited in claim 1, wherein said step b) includes the steps of:determining whether the second call access signal from the mobilestation is acquired by the preamble access window; and accessing thesecond call access signal to the preamble access window by delaying thesecond call access signal if the second call access signal is notacquired.
 4. The method as recited in claim 3, wherein the second callaccess signal is delayed by a feedback delay.
 5. A method for expandingcell coverage in a code division multiple access (CDMA) mobilecommunication system including at least a base station and a pluralityof mobile stations, the method comprising the steps of: a) expanding alength of a current preamble access window to a predetermined value; b)determining the mobile station is within a cell coverage covered by thepreamble access window; c) shifting the preamble access window byadvancing transmission time of a transmission signal by a first delaytime in order to acquire at least a mobile station when the mobilestation is out of the cell coverage; and d) delaying a second callaccess signal from a mobile station at a short distance in order toacquire the second call access signal, wherein the transmission time ofthe transmission signal is advanced by delaying the transmission signalwhen being PN modulated; and wherein the first delay time issubstantially equal to the second delay time, and the first delay timeis expressed as:${{{first}\mspace{14mu}{delay}\mspace{14mu}{{time}({\mu s})}} = {\frac{{{Radius}({Km})} \times 2}{3 \times 10^{5}} \times 10^{6}}},$where Radius is a length of the expanded radius.
 6. The method asrecited in claim 5, wherein said step d) includes the steps of:determining whether the second call access signal is acquired by thepreamble access window; and accessing the second call access signal tothe preamble access window by delaying the second call access signal ifthe second call access signal is not acquired.
 7. The method as recitedin claim 6, wherein the second call access signal is delayed by afeedback delay.
 8. An apparatus for expanding cell coverage in a codedivision multiple access (CDMA) mobile communication system including aplurality of mobile stations and at least a base station, comprising:transmitting means for transmitting a transmission signal of whichtransmission time is advanced by a first delay time in order to acquirethe transmission signal from a mobile station at a remote distance; andreceiving means for receiving and delaying a received signal from amobile station at a short distance by a second delay time in order toacquire the received signal, wherein the transmission time of thetransmission signal is advanced by delaying the transmission signal whenbeing PN modulated; and wherein the first delay time is substantiallyequal to the second delay time, and the first delay time is expressedas:${{{first}\mspace{14mu}{delay}\mspace{14mu}{{time}({\mu s})}} = {\frac{{{Radius}({Km})} \times 2}{3 \times 10^{5}} \times 10^{6}}},$where Radius is a length of the expanded radius.
 9. The apparatus asrecited in claim 8, wherein said transmitting means includes: means forgenerating a call access signal; means for modulating the first callaccess signal on basis of pseudo noise signal and generating thetransmission signal; and means for advancing the transmission signal bythe first predetermined time by controlling the pseudo noise signal. 10.The apparatus as recited in claim 8, wherein said receiving meansincludes: means for delaying the received signal by a firstpredetermined time; and means for demodulating the delayed receivedsignal and restoring the call access signal.
 11. The apparatus asrecited in claim 10, wherein the second call access signal is delayed bya feedback delay.
 12. A method for expanding cell coverage applied to acode division multiple access (CDMA) mobile communication system, themethod comprising the steps of: a) expanding a length of a currentpreamble access window to a maximum value; b) setting operationparameters related to time advance in order to acquire a first callaccess signal from a mobile station at a remote distance when the mobilestation is out of the cell coverage; and c) acquiring a second callaccess signal from a mobile station at a short distance within thepreamble access window by delaying the second call access signal by apredetermined delay time, wherein the transmission time of thetransmission signal is advanced by delaying the transmission signal whenbeing PN modulated, and wherein the predetermined delay time isexpressed as:${{{first}\mspace{14mu}{delay}\mspace{14mu}{{time}({\mu s})}} = {\frac{{{Radius}({Km})} \times 2}{3 \times 10^{5}} \times 10^{6}}},$where Radius is a length of the expanded radius.
 13. The method asrecited in claim 12, wherein said step c) includes the steps of:determining whether the second call access signal is acquired within thepreamble access window; accessing the second call access signal to thepreamble access window by delaying the second call access signal if thesecond call access signal is not acquired; and accessing the second callaccess signal to the preamble access window if the second call accesssignal is acquired.
 14. The method as recited in claim 13, wherein theoperation parameters include a preamble pseudo noise off set used forexpanding the cell coverage.
 15. The method as recited in claim 13,wherein the operation parameters include a transmission time advancefine adjust value used for expanding the cell coverage.
 16. A method forexpanding cell coverage applied to a code division multiple access(CDMA) mobile communication system, the method comprising the steps of:a) expanding a length of a current preamble access window for accessprobe to a maximum value; b) shifting the preamble access window bysetting operation parameters in order to acquire a first call accesssignal from a mobile station at a remote distance when the mobilestation is out of the expanded preamble access window; and c) acquiringa second call access signal from a mobile station at a short distancewithin the preamble access window by delaying the second call accesssignal by a predetermined delay time, by using a feedback delay whichmakes the second call access signal have a different delay value onbasis of a distance between a location of the mobile station and alocation on a cell radius, wherein the transmission time of thetransmission signal is advanced by delaying the transmission signal whenbeing PN modulated, and wherein the predetermined delay time isexpressed as:${{{first}\mspace{14mu}{delay}\mspace{14mu}{{time}({\mu s})}} = {\frac{{{Radius}({Km})} \times 2}{3 \times 10^{5}} \times 10^{6}}},$where Radius is a length of the expanded radius.
 17. The method asrecited in claim 16, wherein said step a) includes the steps of:initializing a delay value and a number of delay of the feedback delay;and setting the length of the preamble access window as a maximum value.18. The method as recited in claim 16, wherein the cell radius isexpanded 45 Km or more in said step b).
 19. The method as recited inclaim 16, wherein said step c) includes the steps of: receiving thesecond call access signal; delaying the second call access signal for apredetermined time by feeding back the second call access signal by acertain number; determining whether the second call access signal or thedelayed signal are acquired within the preamble access window;performing a call access if the second call access signal or the delayedsecond call access signal is acquired within the preamble access window;and stopping a call access if not.
 20. The method as recited in claim19, wherein the operation parameters include a preamble pseudo noiseoffset used for expanding the cell coverage.
 21. The method as recitedin claim 19, wherein the operation parameters include a transmissiontime advance fine adjust value used for expanding the cell coverage. 22.The method as recited in claim 19, wherein the second call access signalis substantially delayed for 20 μs by the feedback delay.
 23. Amachine-accessible medium for expanding cell coverage applied to a codedivision multiple access (CDMA) mobile communication system, themachine-accessible medium containing instructions that, when executed,cause a machine to: a) expand a length of a current preamble accesswindow to a maximum value; b) set operation parameters in order toacquire a first call access signal from a mobile station at a remotedistance when the mobile station is out of the cell coverage; and c)acquire a second call access signal from a mobile station at a shortdistance within the preamble access window by delaying the second callaccess signal, wherein the transmission time of the transmission signalis advanced by delaying the transmission signal when being PN modulated;and wherein the predetermined delay time is expressed as:${{{first}\mspace{14mu}{delay}\mspace{14mu}{{time}({\mu s})}} = {\frac{{{Radius}({Km})} \times 2}{3 \times 10^{5}} \times 10^{6}}},$where Radius is a length of the expanded radius.
 24. Amachine-accessible medium for expanding cell coverage applied to a codedivision multiple access (CDMA) mobile communication system, themachine-accessible medium containing instructions that, when executed,cause a machine to: a) expand a length of a current preamble accesswindow for access probe to a maximum value; b) shift the preamble accesswindow by setting operation parameters in order to acquire a first callaccess signal from a mobile station at a remote distance when the mobilestation is out of the expanded preamble access window; and c) acquire asecond call access signal from a mobile station at a short distancewithin the preamble access window by delaying the second call accesssignal by a feedback delay which makes the second call access signalhave a different delay value on basis of a distance between a locationof the mobile station and a location on a cell radius, wherein thetransmission time of the transmission signal is advanced by delaying thetransmission signal when being PN modulated; and wherein thepredetermined delay time is expressed as:${{{first}\mspace{14mu}{delay}\mspace{14mu}{{time}({\mu s})}} = {\frac{{{Radius}({Km})} \times 2}{3 \times 10^{5}} \times 10^{6}}},$where Radius is a length of the expanded radius.